WELLINGTON, April 5 (Reuters) - Floods and landslips blocked roads across New Zealand's north island on Wednesday after it was swept overnight by the tail-end of powerful Cyclone Debbie, which has left a trail of destruction in Australia.

In Auckland, the north island's biggest city, a cliff collapsed on to an apartment block and more than 1,000 homes were without power. A state of emergency was declared in two other cities as river levels rose amid record rainfall.

No fatalities have been reported, but authorities warned that a lull in the rains early on Wednesday would not last. A renewed downpour over the next 24 hours brings the risk of flash flooding in areas still recovering from a devastating earthquake in November.

"Our ground saturation levels are very high which means our water table is elevated, therefore isolated flooding is highly likely," Civil Defence Controller Garry Towler said in an emergency statement.

New Zealand's mountainous terrain makes its roads susceptible to landslides and many regions are still recovering from November's 7.8-magnitude quake. Two cities were on alert for possible mandatory evacuation later on Wednesday.

Cyclone Debbie, a category four storm, one short of the most powerful level five, pounded Australia's Queensland state last week, smashing tourist resorts, bringing down power lines and shutting down coal mines.

Australian police on Tuesday retrieved three bodies from a car beneath floodwaters in the swollen Tweed River of New South Wales state, taking to six the number of people killed since the storm hit.

In Australia the disaster zone stretched 1,000 km (600 miles) from Queensland's tropical resort islands and Gold Coast tourist strip to the farmlands of New South Wales. About 20,000 homes were still without power on Tuesday, as flood waters continued to rise in some areas.

Residents of Rockhampton, in central Queensland, were advised to seek higher ground as the rising Fitzroy River approaches a peak forecast for Thursday. Australian insurers have declared the event a catastrophe, with state officials saying recovery and repairs will take months. (Editing by Andrew Roche)

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